Redundant Aquarium Filtration

Using More than One Filter on Your Fish Tank

(The First Tank Guide)

What Is Redundant Filtration?

The concept of redundant filtration is using two or more different
filters on an aquarium, rather than just using one filter on its
own.

Don't Multiple Filters "Compete" on the Fish Tank?

It is true that if you use more than one filter on your fish tank,
neither of those filters will perform as efficiently as it would if it
were the only filter on the aquarium. This is expected, but is not a
problem.

Each filter is less efficient because of some of the restrictions
placed on filtration systems by water volume and surface area. Both
water volume and surface area will influence the density of waste and
of oxygen in the water, and this will directly impact the efficiency
of the biological filter over
all. Because of the limitations on available oxygen in the water and
the rate at which waste is produced and delivered to the filter, adding
more filters will not increase the maximum
population for your aquarium. This parallels the rationale behind
not using bacteria boosters in an
established fish tank.

Should I Use Redundant Filtration on My Fish Tank?

I almost always recommend using redundant filtration on your fish tank. The primary reason I recommend this is
that if you have a filter fail for whatever
reason, you have a backup in place that is already cycled and nearly capable of handling the load
in your fish tank immediately. This can buy you some time to get to
your local pet shop and get the parts you
need to fix your other aquarium filter or to get a replacement for the
failed filter, without leaving your tank unfiltered while you are
doing this.

Why Not Use Multiple Filters on an Aquarium?

The most common reason I give for not recommending
redundant filtration on a tank is if one or more of the alternate
filtration methods is not appropriate for that tank or setup. That is
not a recommendation against redundant filtration, but rather a concern
related to one of the types of filters and the aquarium setup. Factors
from location of the tank to the fish or animals you are keeping can
impact whether one form of filtration or another is appropriate for
your fish tank.

Will Adding Another Filter Increase My Maximum Tank
Population?

The capacity of a biological
filter, provided that you have sufficient filtration to support
enough biological filtration for a tank the size of yours, is
determined more by water volume and available oxygen than by the
number or configuration of the filters
being used. So, if you have a 50 gallon aquarium and a filter for a 30
gallon tank, your population will be limited by the undersized
filter. However, if you have a 30 gallon aquarium with enough
filtration for a 50 gallon aquarium, that fish tank really has the
same maximum population as a 30 gallon
aquarium with filtration appropriate to a 30 gallon aquarium.

An Example of Redundant Aquarium Filtration:

If you have a 120 gallon fish tank, and you have a power filter that is designed to support your
120 gallon tank, and you use only the power filter, you should have
enough biological filtration to support your 120 gallon tank.

On the other hand, if you have a canister
filter designed for your 120 gallon filter, and you use your
canister filter, and you use a good biological filtration media in it
so that it can provide biological
filtration for your 120 gallon tank, you will also have enough
biological filtration to support your 120 gallon tank.

If, on the other hand, you use both of these aquarium filters on
the same 120 gallon fish tank, you will have enough biological
filtration to support your 120 gallon tank - not enough to support a
240 gallon tank.

However, if you were to use these same two aquarium filters on a
180 gallon fish tank, you should be able to provide enough biological
filtration for that 180 gallon aquarium.

The filtration capacity increases with multiple filters, but only
if the water volume also increases.

Are There Cases Where the Filtration Will Not Scale?

However, as always, there are exceptions to this rule:

If there is an under gravel filter
involved, the filtration may not scale. Under gravel filters that do
not effectively cover the entire bottom of the tank and efficiently
use the entire substrate as a filter can have the negative side effect
like creating anaerobic pockets in the portions of the gravel that they
do not circulate water through. These anaerobic pockets can produce
toxic gases that could pose a health threat to your fish. So, it is
important to remember that if you are using an under gravel filter,
the filter should cover as much of the bottom of the tank as possible
to provide efficient filtration.

If the filters in question are minimal to the smaller sized tank
(in this case the 120 gallon tank) they will be insufficient to the
larger tank, because the shortcomings can be compounded and it is
likely that the water will not be sufficiently circulated. You need to
be sure that among your filters you at least turn over the entire tank
volume 3-5 times per hour.

What this example is intended to demonstrate is that adding
filtration to the tank, provided that the initial filtration was
sufficient to the tank, does not increase the tank holding capacity in any significant way, nor
does it use any one of the filter elements to its maximum capacity or
efficiency. However, redundant filtration on your fish tank will be
highly beneficial in any case where you need to disable one of the
filters for a period due to a mechanical failure or due to medication.

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